The present invention is directed towards biaxially-oriented films of polyethylene terephthalate containing no more than 3 wt. % of comonomer having superior hydrolytic and thermal stability. Film compositions of the present invention provide extended service lifetime when used as insulation in electrical motors.
Transition from traditional chlorofluorocarbon refrigerants to new, non-ozone depleting alternatives has placed new performance demands on the materials and components traditionally used in refrigeration systems. The reliability of electric motors used in hermetic environments is especially critical. The motor assembly utilizes polyethylene terephthalate film as slot liners, wedges, and interphase insulation which must function while immersed in refrigerant fluid and lubricating oil. Film insulation used in compressor motors must withstand this closed environment for years without loss of physical or electrical properties. Elevated temperature inside the sealed compressor promotes hydrolysis and enhances film embrittlement.
Harrington and Ward (ASHRAE Journal, pgs. 75 to 78, April 1959) have shown that the time to brittleness is correlated to the film exposure temperature and the partial pressure of moisture in the hermetic environment. McMahon et al. (Journal of Chemical Engineering Data, Vol. 4 (1), pgs. 57 to 78, January 1959) have shown that polyethylene terephthalate film is no longer functional as a reliable electrical insulator when the intrinsic viscosity (a measure of molecular weight) has decreased to a level of about 0.30 to 0.33 at which time the film has embrittled and cannot be folded upon itself without cracking.
Those skilled in the art recognize that selection of polymer composition and processing conditions during biaxial orientation and heatsetting establish the structure and, therefore, the properties of oriented polyethylene terephthalate films. Gohil in the Journal of Applied Polymer Science, Vol. 52, pgs. 925 to 944 (1994) discloses broad relationships between structure and properties of polyethylene terephthalate films of conventional molecular weight having an intrinsic viscosity of about 0.55. Gohil proposes a model which relates microstructural reorganization within the film during heat treatment in the temperature range of 100.degree. C. to 240.degree. C. to transitional changes in film properties.
U.S. Pat. No. 3,432,591 to Heffelfinger discloses a biaxially-oriented polyethylene terephthalate film having an intrinsic viscosity greater than 0.82 and specific orientation of the (100) crystal plane relative to the plane of the film for use as electrical insulation. Heffelfinger teaches a preferred film density range of 1.37 to 1.40 g/cc obtained by selecting processing conditions for stretching and heat-setting of the film and the importance of film planarity, i.e., orientation of the (100) crystal plane which contains the benzene ring.
Although it is known in the prior art to introduce an end capping agent as an additive into polyethylene terephthalate film for the purpose of increasing hydrolytic stability, the use of such additive lies outside the scope of the present invention. An end capping agent (such as carbodiimide) reacts with the end groups of the polyethylene terephthalate to account for the stability increase. However, the disadvantage is that unreacted additive will migrate to the film surface or be extracted from the film over a prolonged time period.
In specific applications of the polyethylene terephthalate film such as in motor insulation such migration and/or extraction can be considered unacceptable in certain instances.
Therefore, a need exists for a polyethylene terephthalate film having prolonged hydrolytic stability which does not employ an end capping additive.